The present invention relates generally to a circuit for combining and dividing high frequency power, and more particularly to a microwave power combiner/divider for use in a microwave communication apparatus, etc.
There is known a power combiner/divider for use in a microwave band, which comprises as a basic structure a so-called Wilkinson type power combiner/divider circuit. This circuit is advantageous in that the circuit configuration is simple and it has an impedance conversion function. By virtue of the advantage, this power combiner/divider circuit is widely used, for example, as an output matching circuit in a high output power amplifier.
FIG. 1 shows an equivalent circuit of a conventional Wilkinson type power combiner/divider.
As is shown in FIG. 1, one end of a first transmission line 11 is used as a first signal port (division port) 21, and one end of a second transmission line 12 is used as a second signal port (division port) 22.
The other end of the first transmission line 11 and the other end of the second transmission line 12 are connected at a third signal port (combining port) 23. The first signal port 21 and second signal port 22 are connected via a resistor (isolation resistor) 31.
The length of each of the first and second transmission lines 11 and 12 is set at about 1/4 of the wavelength of the input signal. The characteristic impedance of each transmission line 11, 12 and the value of the resistor 31 are determined by the impedances of the first to third signal ports 21, 22 and 23.
It is known that the characteristic impedance should ideally be set at (2R.sub.L .multidot.R.sub.g).sup.1/2 and the resistance at 2R.sub.L in the case of a circuit wherein the impedances of the first and second signal ports 21 and 22 (division ports) and the impedance of the third port 23 (combining port) are set at R.sub.L and R.sub.g respectively and a signal input from the third signal port 23 is divided into equal-level signals to the first and second signal ports 21 and 22.
When the circuit shown in FIG. 1 is actually used, power amplifier circuits, for example, are connected to the first and second signal ports 21 and 22. As a matter of course, when such amplifier circuits are connected, the first signal port 21 and second signal port 22 need to be disposed away from each other at a predetermined distance.
Although the resistor connected between the first signal port 21 and second signal port 22 should ideally be a lumped-parameter device, as mentioned above, the resistor is influenced by a parasitic element occurring due to the increased distance between the first and second signal ports 21 and 22.
FIG. 2 shows an equivalent circuit in a case where there is a distance between the first and second signal ports 21 and 22.
In the circuit diagram, the influence of the parasitic element at the resistor can be expressed by transmission lines 32 and 33 connected to the resistor 31. Suppose that in the equivalent circuit of the power combiner/divider circuit, as shown in FIG. 2, microstrip lines are provided on a GaAs semi-insulative substrate with a thickness of 30 .mu.m, and the distance between the first signal port (division port) 21 and second signal port (division port) 22 is 530 .mu.m and the width of each microstrip line (i.e. first and second transmission lines 11 and 12) is 150 .mu.m. In this case, two wiring elements having a length of 190 .mu.m are needed to connect the resistor 31. FIG. 3 shows a simulation result, in which a parasitic effect in this case is taken into account.
The abscissa in FIG. 3 indicates the frequency (GHz) and the ordinate indicates the isolation and return loss (dB). The center frequency is 29.5 GHz, the impedance of each of the division ports (first and second signal ports) and that of the combining port (third signal port) are respectively 3.OMEGA. and 50.OMEGA., and the isolation resistance of 6.OMEGA. is connected to the microstrip lines with a width of 20 .mu.m.
As is understood in FIG. 3, the return loss and isolation of the division port, which can be obtained, are at most about 7 dB at the center frequency.
As has been described above, the effect of the resistor itself is considerably deteriorated by the addition of the parasitic element to the resistor portion, and the isolation between the first signal port 21 and second signal port 22 is degraded.